Segmented structures are conventionally used in planar waveguide structures to act as fiber-to-waveguide couplers (FWC), Bragg gratings, or other such structure, whereby the geometry of the segmented structures is chosen to optimize some feature in transmission. For example, an FWC gradually enables an optical mode to expand or contract to match the mode of an optical fiber to the mode most conveniently carried within the planar waveguide structure. The reflection from the interface of any element of a segmented structure is generally very small; however, if many segmented structures are employed, the reflection from each interface of each segment will add to the reflection of other interfaces to produce a potentially large cumulative back reflection. In the case where the segments are “random”, or of no particular period, the individual segment reflections will accumulate to a relatively wavelength independent back reflection. In the case where the segments are periodic, the cumulative effect will show strong back reflections at specific wavelengths, and weaker reflections in between those wavelengths. Cumulative back reflections exceeding approximately −35 dB (approximately 0.03%) can be unacceptable in many waveguide applications, e.g. if the waveguide is receiving light from a laser. FIG. 1 illustrates a conventional segmented waveguide structure 1, in which the segments 2 are made of core material (dashed filled), and are surrounded by cladding material 3 for guiding light 4 between a continuous waveguide section 5 and an edge 6 of the structure 1, wherein the segments 2 have progressively smaller widths. Examples of devices including segmented waveguides are illustrated in U.S. Pat. No. 5,745,618 issued Apr. 28, 1998 to Li; U.S. Pat. No. 6,892,004 issued May 10, 2005 to Yu; U.S. Pat. No. 7,006,729 issued Feb. 28, 2006 to Wang et al; U.S. Pat. No. 7,130,518 issued Oct. 31, 2006 to Yamazaki et al; and U.S. Pat. No. 7,212,709 issued May 1, 2007 to Hosai et al.
The segmented waveguides 2 are positioned in transition areas to provide mode expansion or mode contraction depending upon which direction the light 4 travels. The mode expansion and contractions are used to gradually match an optical field of an optical signal in the waveguide section 5 to optical fields of corresponding optical signals in the adjacent guiding structures optically coupled to the segmented waveguides 2, e.g. optical fibers, slab waveguides etc, connected to the edge 6.
Unfortunately, there is a reflection from each interface between the core segments 2 and the cladding 3, which can combine coherently when the segments 2 are positioned periodically or quasi-periodically, e.g. spaced at a distance equal to the wavelength (λ) of the transmitted light or multiples thereof. In FIG. 2, a conventional method of reducing back reflections is demonstrated in a randomly offset, e.g. not periodic, segmented device 7 in which each of the aforementioned segments 2, shown in solid outline, is moved in some random but small amount from its nominal location, resulting in repositioned segments 2′, shown in phantom outline. The feedback from randomly repositioned segments 2′ will likely not add together coherently after repositioning, thereby suppressing some back reflection; however, randomizing has limited benefits, and provides only from 10 dB to 20 dB of back reflection suppression. Alternatively, the widths of individual segments might vary to achieve the same randomization effect (not shown here).
An object of the present invention is to overcome the shortcomings of the prior art by providing a means for modifying and, when necessary, substantially suppressing cumulative back reflection from segmented planar waveguide structures.